Tin Bromido Aluminate Networks with Bright Luminescence

Abstract The novel tin bromido aluminates [Sn3(AlBr4)6](Al2Br6) (1), Sn(AlBr4)2 (2), [EMIm][Sn(AlBr4)3] (3) and [BMPyr][Sn(AlBr4)3] (4) ([EMIm]: 1‐ethyl‐3‐methylimidazolium, [BMPyr]: 1‐butyl‐1‐methyl‐pyrrolidinium), are obtained from a ionic‐liquid‐based reaction of AlBr3 and SnCl2 or SnBr2, resulting in colorless and transparent crystals. 1 contains a neutral, inorganic ∞ 3[Sn3(AlBr4)6] network filled with intercalated Al2Br6 molecules. 2 represents a 3D structure isotypic to Pb(AlCl4)2 or α‐Sr[GaCl4]2. 3 and 4 exhibit infinite ∞ 1[Sn(AlBr4)3]n− chains that are separated by the voluminous [EMIm]+/[BMPyr]+ cations. All title compounds contain Sn2+ coordinated by AlBr4 tetrahedra, resulting in chains or 3D networks. Moreover, all title compounds show photoluminescence due to Br−→Al3+ ligand‐to‐metal charge‐transfer excitation, followed by 5s 2 p 0←5s 1 p 1 emission on Sn2+. Most surprisingly, the luminescence is highly efficient (quantum yield >50 %). Specifically, 3 and 4 exhibit outstanding quantum yields of 98 and 99 %, which are the highest values observed for Sn2+‐based luminescence so far. The title compounds have been characterized by single‐crystal structure analysis, elemental analysis, energy‐dispersive X‐ray analysis, thermogravimetry, infrared and Raman spectroscopy, UV‐Vis and photoluminescence spectroscopy.


Elemental analysis (C/H/N/S analysis) was performed via thermal combustion with an Elementar
Vario Microcube device (Elementar, Germany) at a temperature of 1,100 °C. The samples were prepared in the glove-box and transferred to the elemental analysis in air-tight containers. The samples were either combusted in tin or silver capsules, which are both temporarily air-tight.
Thermogravimetry (TG) was carried out with a Netzsch STA 449 F3 Jupiter device using α-Al2O3 as crucible material and reference. Buoyancy effects were corrected by baseline subtraction of a blank measurement. The samples were measured under dried nitrogen up to 800 °C with a heating rate of 10 K/min. The samples were prepared by filtration through a glass filter under argon. Inside an Arfilled glovebox, the sample was transferred to an α-Al 2 O 3 crucible with a lid. Afterwards the crucible was transferred to the thermogravimetry in an air-tight container and placed inside the device with -S3-N2 flux. Immediately thereafter, the whole measurement chamber was evacuated and refilled with dried nitrogen before staring the measurement.
Fourier-transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer (Bruker). The samples were measured as pellets in KBr. Thus, 300 mg of dried KBr and 0.5-1.0 mg of the title compound were carefully pestled together and pressed to a thin pellet inside a glovebox. These pellets were placed in the sample holder of the device inside an Ar-filled glovebox and transferred in an air-tight container into the spectrometer. The sample holder was placed inside the already N2-filled measurement chamber with nitrogen flux.
Raman spectroscopy. Raman spectra were recorded on a Bruker Raman microscope Senterra II using its 532 nm laser. The combination of the CCD detector and the grating (400 lines/mm) leads to a resolution of 4 cm −1 . The samples consisted of selected single crystals, which were cleaned from the ionic liquid by immersing in perfluorinated polyether and crushing into smaller pieces. Thereafter, these samples were fixed inside an Ar-filled glovebox on the inner side of an Ar-flushed and preheated glass tube on a small area of approximately 1 mm 2 in size and measured within the closed glass tube.
Optical spectroscopy (UV-Vis) of powder samples was recorded on a Shimadzu UV-2700 spectrometer, equipped with an integrating sphere, in a wavelength interval of 250-800 nm against BaSO4 as reference. 10 mg of sample were pestled together with dried BaSO4 and filled into an airtight sample holder inside an Ar-filled glovebox. Afterwards, the Ar-filled sample holder was transferred to the spectrometer for measurement.

Structural Properties
For single crystal structure analysis, suitable crystals were manually selected, covered by inert-oil (perfluoropolyalkylether), and placed on a micro gripper (MiTeGen). Data collection for 2-4 was performed at 200 or 213 K on an IPDS II image-plate diffractometer (Stoe, Darmstadt) using Mo-Kα radiation (λ = 0.71073 Å, graphite monochromator). Data collection for 1 was performed at 180 K on a Stoe StadiVari Diffractometer with Euler geometry (Stoe, Darmstadt) using Ga-Kα radiation (λ = 1.34143 Å, graded multilayer mirror as monochromator). Detailed information on crystal data and structure refinements are listed in Table S2.
-S4- To illustrate the 3D network structure of 1, different views of the unit cell are shown ( Figure S1).
Moreover, the coordination of Sn 2+ with [AlBr4]and the resulting connectivity for the different Sn 2+ sites of 1 are shown ( Figure S2). To illustrate the connectivity in 1 and 2, unit cells in a wireand-sticks model are shown ( Figure S3). Here, Sn-Br distances above the longest distance in SnBr2 are illustrated as dotted lines. To compare the coordination of Sn 2+ and Sn-Br distances, coordination polyhedra and the respective distances are shown for all title compounds ( Figure S4).

Chemical Composition
To validate the chemical composition of the title compounds, elemental analysis (C,H,N analysis), energy-dispersive X-ray spectroscopy (EDXS) and thermogravimetry (TG) were performed.
According to EA, minor amounts of C/H/N are found for 1 and 2, which can be attributed to adhered ionic liquid on the crystal facets. For 3 and 4, the values for C and N are slightly higher than calculated, which can be again attributed to adhered ionic liquid on the crystal facets (Table S3).
Furthermore, it needs to be noticed that the hydrogen content obtained is too low compared to the calculated values of the [EMIm]/[BMPyr]-containing compounds 3 and 4 (Table S3). We assume the lack of hydrogen is caused by the evaporation of HBr, which cannot be detected with our equipment.
To address this limitation, the tin capsules, usually applied to perform EA, were replaced by silver capsules, which prevent HBr evolution due to the formation of AgBr. As a result of EA performed in silver capsules, the obtained H values were indeed higher compared to those obtained with tin capsules (Table S3).
EDXS confirms the presence of Sn and Al/Br ( Figure S6a-d). Al and Br cannot be reliably distinguished due to the similar energy of the Al-Kα emission (1.486 eV) and the Br-Lα emission (1.480 eV). Moreover, single crystals of the title compounds decompose rapidly in vacuum under electron bombardment (30 kV) due to Al2Br6 release ( Figure S6e). As a result, scanning electron microscopy (SEM) after EDXS analysis shows a highly porous remain of a former single crystal. The evaporation of Al2Br6 is also confirmed by TG as well as by literature data (sublimation of Al2Br6 starting at 28 °C). [S1] Due to total decomposition of the title compounds, TG allows to quantify the chemical composition and to study the thermal properties (see main paper, Figure S7).
-S8-  In addition, PXRD measurements were tried. However, due to their moisture sensitivity, the samples need to be filled and measured in glass capillaries sealed under argon. Due the low melting, the powders become liquid when mortaring. Even after cooling, the powder particles glue together and form even larger particles. Therefore, we were not able to obtain any suitable diffractograms.

Material Properties
To examine the optical properties, ultraviolet-visible (UV-Vis) spectroscopy of all title compounds was performed ( Figure S8). Here, absorption below 420 nm (1), 450 nm (2), and 400 nm (3, 4) is observed, which can be related to a Br --to-Al 3+ ligand-to-metal charge transfer. The absorption is more-or-less similar to the binary halide AlBr3. Below 250 nm, finally, valence-band to conductionband absorption occurs.
In regard of the excitation process, in general, there are four options: (1) Valence-band (Vb) to conduction-band (Cb) excitation: Here, a comparison with the binary phases SnBr2 and AlBr3 is indicative. For SnBr2, Vb → Cb excitation is observed < 400 nm ( Figure   S8). For AlBr3, Vb → Cb excitation is observed < 230 nm ( Figure S8). In both cases the very intense and very broad absorption points to Vb → Cb excitation. The respective wavelengths are in accordance with the literature and not relevant for the excitation of the title compounds.
(2) Br -→ Al 3+ LMCT excitation: For AlBr3, Br -→ Al 3+ LMCT excitation is observed at 240-370 nm ( Figure S8). Shape and position fit very well with all title compounds, which indicates the Br -→ Al 3+ LMCT as the origin of the excitation of the title compounds. -S10- (3) Br -→ Sn 2+ LMCT excitation: The Br -→ Sn 2+ LMCT excitation is expected to be at significantly higher energy/lower wavelength as compared to the Br -→ Al 3+ LMCT excitation (due to the lower charge of Sn 2+ in comparison to Al 3+ ). Therefore, this transition is not relevant here.
(4) s → p transition on Sn 2+ : This transition cannot be excluded completely since it is to be expected in a similar wavelength regime as the Br -→ Al 3+ LMCT excitation. The excitation spectra of the title compound at 240-370 nm, however, look very similar to AlBr3, so the Br -→ Al 3+ LMCT excitation seems most probable.